A fluid pressure circuit for supplying compressed air from a compressed air source to a fluid pressure operated unit such as a pneumatic pressure cylinder is provided with control units such as a pressure control valve for adjusting the pressure of compressed air and a directional control valve for switching the flow paths, and a fluid pipe such as a tube is used to connect respective members, which constitute the fluid pressure circuit, to each other. The fluid pipe for connecting the respective members which constitute the fluid pressure circuit is used also in the fluid pressure circuit for supplying liquid pressurized by the pressurization source to the fluid pressure operated unit. The fluid pipe is used also in the fluid pressure circuit for supplying negative pressure air from a negative pressure air source constituted of a vacuum pump and the like to a negative pressure device such as a vacuum pad. The fluid pipe which guides working fluid such as positive pressure air, negative pressure air and liquid includes tube and hose formed of flexible material such as rubber and soft resin and pipes formed of metal or hard resin.
Such a fluid pipe is connected to a control unit or a fluid pressure operated unit through a pipe joint, and as the pipe joint, for example, a screw-in pipe joint, a flareless pipe joint and a flared pipe joint are used for the connection of metallic fluid pipes, and a flareless pipe joint, an instant pipe joint (quick pipe joint) and a barb-type pipe joint are used for the connection of non-metallic pipes, and the fluid pipe is detachably attached so that it can be removed from the pipe joint. Further, the pipe joint includes a quick joint which has a male screw portion to be screwed to a member constituting a pneumatic pressure circuit and to which the fluid pipe is connected detachably.
When the fluid pressure operated unit is actuated in the fluid pressure circuit, fluid flows in the fluid pipe. On the other hand, although no fluid flows in the fluid pipe when a drive unit is stopped, the interior of the fluid pipe is filled with pressurized fluid or negative pressure fluid in some cases. If the fluid pipe provided in the fluid pressure circuit is removed by mistake from the control unit or the pipe joint in the state where the pressurized fluid is filled, pressurized fluid supplied from the fluid pressure source is spouted out.
In order to make it possible to externally confirm whether or not any fluid flows in the fluid pipe connected to the pipe joint, Patent Document 1 has described a pipe joint which has an indicating portion provided so as to project outwardly from an end portion of a L-shaped joint main body or an indicating portion provided outside a straight joint main body so as to project outwardly in a diameter direction of the joint main body. Also, Patent Document 2 has described a pipe joint having a piston provided to a joint main body so as to project outwardly in a diameter direction of the joint main body. On the other hand, Patent Document 3 has described a pressure gauge in which a piston is incorporated movably in an axial direction in a transparent straight tube so that a flow and pressure condition of fluid can be externally confirmed depending on the position of the piston.    Patent Document 1: Japanese Utility Model Application Laid-Open Publication No. 2-85096    Patent Document 2: Japanese Utility Model Application Laid-Open Publication No. 2-105693    Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2001-74762